## Subtitles section Play video

• To send an electrical signal, we need two wires.

• Since two metal conductors are by definition a capacitor,

• this means that each section of our transmission line has some capacitance.

• And since a loop of wire is by definition an inductor,

• each section of our line also has some inductance.

• Suppose we close a switch applying a constant DC voltage across our two wires.

• This capacitance and inductance is what prevents

• the signal from travelling instantaneously.

• Therefore, we can never eliminate this capacitance and inductance completely

• because this would then violate Einstein's Theory of Relativity,

• which states that information can never travel faster than the speed of light.

• These capacitors and inductors shown

• are not components that we deliberately add to a circuit,

• but are an inherent part of all wires that carry electrical signals.

• When electric circuits appear to behave

• in unpredictable ways that seem to violate the laws of physics,

• this is often due to the capacitance and the inductance of the wires.

• In this instance, for example, the signal keeps bouncing back and forth

• between the beginning and the end this transmission line.

• With nothing connected to the end of the line,

• the voltage at the end of the transmission line reaches

• double the value of the DC input voltage from the battery,

• and the signal is then reflected back.

• Reflections can also occur at the connection between two transmission lines

• which have different amounts of capacitance and inductance,

• in which case only a portion of the signal's energy is transmitted to the next line,

• and the rest of the energy is reflected back.

• Suppose we connect a short circuit at the end of a transmission line.

• What will happen when this signal reaches this short circuit?

• As always, the speed with which a signal travels depends

• on the capacitance and inductance of each section of the wire.

• When the signal reaches the short circuit, the signal is reflected,

• but with the voltage flipped upside down!

• If the impedance at the end of the line is too low

• it will behave like a short circuit,

• and if the impedance is too high it will behave like an open circuit,

• both of which cause reflections.

• But, if we place just the right amount of impedance at the end of the line

• for the amount of capacitance and inductance in each section of the line,

• then the amount of reflection can be minimized.

• Reducing reflections is desirable, as reflections can

• significantly interfere with the proper operation of a circuit.

• Much more information is available in the other videos on this channel,

To send an electrical signal, we need two wires.

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B1 US signal capacitance circuit transmission line voltage

# Transmission Lines - Signal Transmission and Reflection

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Amy.Lin posted on 2021/01/06
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